Flux-Cored Arc Welding
A metal arc welding process utilizing a tubular wire electrode containing flux or alloying elements inside. This improves arc stability and permits high deposition rates.
Flux-cored wire is basically a tube filled with powders inside. That stuff melts during welding and forms slag and shielding gas, so you get a clean seam without having to haul around a heavy gas cylinder. There are versions with and without gas, depending on what you need for the job. Make sure the wire doesn't get damp; the powder will absorb water and then you'll get pores in the seam. If the settings aren't right, you'll notice it right away from the spatter behavior, so better to experiment a bit and adjust the parameters until the arc runs smoothly. You'll have to knock off the slag properly afterward, otherwise the seam will rust under the crust.